1. Field of the Invention
The present invention relates to injection-moulded printed circuit boards and processes for their production. The essence of the invention is that thermoplastic materials are injection-moulded onto the back of flexible carrier materials, onto which the conductor tracks have been applied by certain fully or semiadditive techniques before or after the injection-moulding onto the back.
Injection-moulding thermoplastic materials onto the back of films to produce decorative shaped articles and also to produce injection-moulded components which carry electrical or electronic circuits and can thus also be called "injection-moulded printed circuit boards" are known.
2. Description of the Related Art
Flexible circuits, that is to say films, which contain conductor tracks on one side or also those on which conductor tracks are located on both sides and if appropriate the two sides are also connected to one another electrically by plated-through holes are thus placed in injection moulds and thermoplastic material is injection-moulded onto the back of them. Printed circuit boards on which the electrical/electonic circuits are located in one plane can also be produced in this manner. Preferably, however, this technique of injection-moulding onto the back of circuits is used to produce three-dimensional "injection-moulded circuits" (EP-A 0,067,902).
The flexible circuits for the production of injection-moulded printed circuit boards can be produced by printing a conductive paste, preferably a paste which contains Ag powder, Cu powder, Ni powder or graphite in the form of the circuit pattern onto a carrier film with the aid of the screen printing technique (WO 87/01557).
Such flexible circuits can likewise be employed for producing injection-moulded printed circuit board which are obtained by subtractive processes. To produce these boards, copper foils are laminated onto carrier films with the aid of an adhesive, the copper surface is provided with an etch resist in the form of the conductor tracks pattern by customary techniques, the non-covered areas of the copper foil are etched away in a customary copper etching bath, and finally the etch resist is stripped off again.
In a particular embodiment of the production of injection-moulded circuits, the copper-laminated film on which the copper surface is partly provided, that is to say in the form of the conductor tracks pattern, with an etch resist is placed in the injection mould and a thermoplastic is injection-moulded onto the back. The conductor tracks are produced on the three-dimensional injection moulding by etching away the copper zones not covered with etch resist (U.S. Patent Specification 4,710,419).
The processes described have various disadvantages:
The conductor tracks, which consist of polymers filled with metal powder, have a relatively low conductivity and cannot be soldered, which greatly limits the use of such circuits. PA1 Flexible circuits which have been produced by subtractive processes from carrier films, preferably polyimide films, laminated with copper foil for materials to be injection-moulded onto their back have the following adverse features: PA1 a. the flexible carrier is printed with a screen printing paste, which contains an activator (that means a metallisation catalyst) in the form of the conductor track pattern, PA1 b. the thinly applied print of the conductor track pattern is conditioned by drying or heat treatment processes for subsequent electroless metallisation, and PA1 c. finally, a copper layer in the form of the printed conductor track pattern in a height of 0.05-10 .mu.m, preferably 0.1-4 .mu.m, is built up in a chemical, electroless copper bath.
The thickness of the copper layer is at least 17.5 .mu.m. Conductor tracks of this or a higher layer thickness can suffer fractures at the corners and edges of the three-dimensional bodies to be produced by material being injection-moulded onto the back. Even one interruption in a conductor track already causes the entire circuit to be incapable of functioning. PA2 The intermediate layer of adhesive (20-30 .mu.m thick) needed for a firmly adhering lamination has lower electrical and thermal properties than the flexible carrier material, preferably Kapton.RTM. film. This has a particularly adverse effect if circuits are to be produced with fine conductor tracks.